Ganymede: Facts About Jupiter's Largest Moon Ganymede: Facts About Jupiter's Largest Moon By Kim Zimmermann | June 15, 2016 10:53am ET MORE NASA's Voyager 2 spacecraft snapped this color image of Jupiter's moon Ganymede, the largest satellite in the solar system, on July 7, 1979 from a distance of 745,000 miles (1.2 million kilometers). Credit: NASA Jupiter's moon Ganymede is the largest satellite in the solar system. Larger than Mercury and Pluto, and only slightly smaller than Mars, it would easily be classified as a planet if were orbiting the sun rather than Jupiter. The moon likely has a salty ocean underneath its icy surface, making it a potential location for life. The European Space Agency plans a mission to Jupiter's icy moons that in 2030, is planned to arrive and put special emphasis on observing Ganymede. Facts about Ganymede Age: Ganymede is about 4.5 billion years old, about the same age as Jupiter .

Distance from Jupiter: Ganymede is the seventh moon and third Galilean satellite outward from Jupiter, orbiting at about 665,000 miles (1.070 million kilometers). It takes Ganymede about seven Earth-days to orbit Jupiter. [ Photos of Ganymede, Jupiter's Largest Moon ] Size: Ganymede's mean radius is 1,635 miles (2,631.2 km). Although Ganymede is larger than Mercury it only has half its mass, classifying it as low density. Temperature: Daytime temperatures on the surface average minus 171 degrees Fahrenheit to minus 297 F, and night temperatures drop to -193C. In 1996, astronomers using the Hubble Space Telescope found evidence of a thin oxygen atmosphere. However, it is too thin to support life as we know it; it is unlikely that any living organisms inhabit Ganymede. Magnetosphere: Ganymede is the only satellite in the solar system to have a magnetosphere. Typically found in planets, including Earth and Jupiter, a magnetosphere is a comet-shaped region in which charged particles are trapped or deflected.

Ganymede's magnetosphere is entirely embedded within the magnetosphere of Jupiter. This montage compares New Horizons' best views of Ganymede, Jupiter's largest moon, gathered with the spacecraft's Long Range Reconnaissance Imager (LORRI) and its infrared spectrometer, the Linear Etalon Imaging Spectral Array (LEISA). Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute Discovery and exploration of Ganymede Ganymede was discovered by Galileo Galilei on Jan. 7, 1610. The discovery, along with three other Jovian moons , was the first time a moon was discovered orbiting a planet other than Earth. Galileo's discovery eventually led to the understanding that planets orbit the sun, instead of our solar system revolving around Earth. Galileo called this moon Jupiter III. When the numerical naming system was abandoned in the mid-1800s, the moon was named after Ganymede, a Trojan prince in Greek mythology. Zeus, a counterpart of Jupiter in Roman mythology, carried Ganymede, who had taken the form of an eagle, to Olympus, where he became a cupbearer to the Olympian gods and one of Zeus' lovers.

Several spacecraft have flown by Jupiter and its moons. Pioneer 10 arrived first, in 1973, followed by Pioneer 11 in 1974. Voyager 1 and Voyager 2 returned striking photos during their flybys. The Galileo spacecraft passed as low as 162 miles (261 km) over the surfaces of the Galilean moons and produced detailed images. The European Space Agency plans a mission called JUICE (JUpiter ICy moons Explorer) that would launch from Earth in 2022 for arrival at Jupiter in 2030. While the mission will look at three moons (Ganymede, Callisto and Europa), Ganymede will be the focus because it shows how icy worlds evolve and could be habitable in general, ESA said on its website. Scientists will try to figure out more about its ocean and icy crust, map its surface in detail, learn about the interior, probe the atmosphere and study the magnetic field.

Ganymede sound and signal illustration Characteristics of Ganymede Ganymede has a core of metallic iron, which is followed by a layer of rock that is topped off by a crust of mostly ice that is very thick. There are also a number of bumps on Ganymede's surface, which may be rock formations. In February 2014, NASA and the United States Geological Survey unveiled the first detailed map of Ganymede in images and a video animation created using observations from NASA's Voyager 1 and Voyager 2 spacecraft, as well as the dedicated Jupiter-orbiting Galileo spacecraft. Ganymede's surface is made up of primarily two types of terrain: about 40 percent is dark with numerous craters, and 60 percent is lighter in color with grooves that form intricate patterns to give the satellite its distinctive appearance. The grooves, which were likely formed as a result of tectonic activity or water being released from beneath the surface, are as high as 2,000 feet and stretch for thousands of miles. A global image mosaic of Jupiter's moon, Ganymede created with images from the Voyager and Galileo missions.

Credit: Wes Patterson It is believed that Ganymede has a saltwater ocean below its surface. In 2015, a study by the Hubble Space Telescope looked at Ganymede's auroras and how they change between Ganymede's and Jupiter's magnetic fields. The “rocking” seen by the auroras gives evidence that the probable ocean underneath is salty , more salty than oceans of Earth, scientists said at the time. Some scientists are skeptical that Ganymede could host life, however. Due to its internal structure, it is believed that the pressure at the base of the ocean is so high that any water down there would turn to ice . This would make it difficult for any hot-water vents to bring nutrients into the ocean, which is one scenario under which scientists believe extraterrestrial life would occur. Additional reporting by contributor Elizabeth Howell. Additional resource Jupiter's Moon Ganymede - Universe Today Universe Today by Matt Williams In 1610, Galileo Galilei looked up at the night sky through a telescope of his own design . Spotting Jupiter, he noted the presence of several “luminous objects” surrounding it, which he initially took for stars.

In time, he would notice that these “stars” were orbiting the planet, and realized that they were in fact Jupiter’s moons – which would come to be named Io , Europa , Ganymede and Callisto . Of these, Ganymede is the largest, and boasts many fascinating characteristics. In addition to being the largest moon in the Solar System, it is also larger than even the planet Mercury. It is the only satellite in the Solar System known to possess a magnetosphere, has a thin oxygen atmosphere, and (much like its fellow-moons, Europa and Callisto) is believed to have an interior ocean. Discovery and Naming: Though Chinese astronomical records claim that astronomer Gan De may have spotted a moon of Jupiter (probably Ganymede) with the naked eye as early as 365 BCE, Galileo Galilei is credited with making the first recorded observation of Ganymede on January 7th, 1610 using his telescope. Together with Io, Europa and Callisto, he named them the “Medicean Stars” at the time – after his patron, the Grand Duke of Tuscany, Cosimo de’ Medici.

Simon Marius, a German astronomer and contemporary of Galileo’s who claimed to have independently discovered Ganymede, suggested alternative names at the behest of Johannes Kepler. However, the names of Io, Europa, Ganymede and Callisto – which were all taken from classical mythology – would not come to formally be adopted until the 20th century. Illustration of Jupiter and the Galilean satellites. Credit: NASA Prior to this, the Galilean Moons were named Jupiter I through IV based on their proximity to the planet (with Ganymede designated as Jupiter III). Following the discovery of the moons of Saturn, a naming system based on that of Kepler and Marius was used for Jupiter’s moons. In Greek mythology, Ganymede was the son of King Tros (aka. Ilion), the namesake of the city of Troy (Ilium). Size, Mass and Orbit: With a mean radius of 2634.1 ± 0.3 kilometers (the equivalent of 0.413 Earths), Ganymede is the largest moon in the Solar System and is even larger than the planet Mercury.

However, with a mass of 1.4819 x 10²³ kg (the equivalent of 0.025 Earths), it is only half as massive. This is due to Ganymede’s composition, which consists of water ice and silicate rock (see below). Ganymede’s orbit has a minor eccentricity of 0.0013, with an average distance (semi-major axis) of 1,070,400 km – varying from 1,069,200 km at periapsis to at 1,071,600 km apoapsis. Ganymede takes seven days and three hours to completes a single revolution. Like most known moons, Ganymede is tidally locked, with one side always facing toward the planet. Its orbit is inclined to the Jovian equator, with the eccentricity and inclination changing quasi-periodically due to solar and planetary gravitational perturbations on a timescale of centuries. These orbital variations cause the axial tilt to vary between 0 and 0.33°.

Ganymede has a 4:1 orbital resonance with Io and a 2:1 resonance with Europa. Ganymede is the largest satellite in our solar system, larger than Mercury and Pluto, and three-quarters the size of Mars. Credit: NASA/JPL Essentially, this means that Io orbits Jupiter four times (and Europa twice) for every orbit made by Ganymede. The superior conjunction between Io and Europa occurs when Io is at periapsis and Europa is at apoapsis, and the superior conjunction between Europa and Ganymede occurs when Europa is at periapsis. Such a complicated resonance (a 4:2:1 resonance) is called the Laplace Resonance . Composition and Surface Features: With an average density of 1.936 g/cm3, Ganymede is most likely composed of equal parts rocky material and water ice. It is estimated that water ice constitutes 46–50% of the moon’s mass (slightly lower than that of Callisto) with the possibility of some additional volatile ices such as ammonia being present.

Ganymede’s surface has an albedo of about 43%, which suggests that water ice makes up a mass fraction of 50-90% the surface. Near-infrared and ultra-violet surveys have also revealed the presence of carbon dioxide, sulfur dioxide, and possibly cyanogen, hydrogen sulfate and various organic compounds. More recent data has shown evidence of salts such as magnesium sulfate and possibly sodium sulfate, which may have originated from the subterranean ocean (see below). Ganymede’s interior appears to be fully differentiated, consisting of a solid inner core made of iron, a liquid iron and iron-sulfide outer core, a silicate mantle, and a a spherical shell of mostly ice surrounding the rock shell and the core. The core is believed to measure 500 km in radius, and has a temperature of about 1500 – 1700 K and pressure of up to 10 GPa. Artist’s cut-away representation of the internal structure of Ganymede, with layers shown to scale. Credit: Wikipedia Commons/kelvinsong The most compelling evidence for the existence of a liquid, iron-nickel-rich core is Ganymede’s intrinsic magnetic field.

The convection in the liquid iron, which has high electrical conductivity, is the most reasonable model of magnetic field generation.The density of the core is believed to be 5.5 – 6 g/cm³, while the silicate mantle has an estimated density of 3.4 – 3.6 g/cm³. This mantle is composed of silicate materials, most likely chondrites and iron. The outer ice shell is the largest layer of all, measuring an estimated 800 km (497 miles) thick. The precise thicknesses of this and other layers in the interior of Ganymede depends on the assumed composition of silicates and amount of sulfur in the core. Scientists also believe that Ganymede has a thick ocean nestled between two layers of ice – a tetragonal layer between it and the core and a hexagonal layer above it. The presence of this ocean has been confirmed by readings taken by orbiters and through studies of how Ganymede’s aurora behaves. In short, the moon’s auroras are affected by Ganymede’s magnetic field, which in turn is affected by the presence of a large, subsurface salt-water ocean. Ganymede’s surface is a mix of two types of terrain.

There’s the very old, highly cratered, and dark regions, and the somewhat younger, lighter regions marked with an extensive array of grooves and ridges. In a way that is similar to Europa, Ganymede’s surface is asymmetric, with the leading hemisphere being brighter than the trailing one. The dark terrain, which comprises about one-third of the surface, is so-colored because the surface ice in these regions contains clays and organic materials. It has been theorized that these have been left behind by impactors, which accords with the fact that impact craters are far more extensive in the areas of dark terrain. Meanwhile, the grooved terrain is believed to be tectonic in nature; which could be due in part to cryovolcanism , but is thought to be mostly the result of tidal heating events. The tidal flexing could have heated the interior and strained the lithosphere, leading to the development of cracks, graben and faults that erased the old, dark terrain on 70% of the surface. Though craters are more common in the darker areas, they are seen all over the surface.

Ganymede may have experienced a period of heavy cratering 3.5 to 4 billion years ago similar to that of the Moon. If true, the vast majority of impacts happened in that epoch, whereas the cratering rate has been much smaller since. Craters on Ganymede are also flatter than those on the Moon and Mercury, which is probably due to the relatively weak nature of Ganymede’s icy crust. Ganymede also has polar caps, likely composed of water frost, which were first seen by the Voyager spacecraft. Since the discovery, several theories have been proposed for their formation, ranging from the thermal migration of water vapor to higher latitudes to plasma bombardment turning the ice brighter. Data obtained by the Galileo spacecraft – which noted a very close correspondence between the polar cap boundary and the boundaries of the moon’s magnetic field – suggests that the latter theory is correct. Atmosphere: Similar to Europa, Ganymede has a tenuous oxygen atmosphere. Also similar to Europa is how the atmosphere formed, which involves water ice on the surface being split into hydrogen and oxygen through interaction with UV radiation, the hydrogen being lost to space and the oxygen being retained.

The surface pressure of this atmosphere is thought to lie within the range of 0.2–1.2 micro Pascals. Montage showing New Horizons’ views of Ganymede taken by it’s infrared spectrometer and LORRI and LEISA instruments. Credit: NASA/Johns Hopkins University APL/SwRI The presence of this atmosphere also causes an airglow effect, a faint emission of light caused by the interaction of atomic oxygen and energetic particles. This effect is not uniformly distributed (as with Europa), but instead causes bright spots to appear above the polar regions – which could be “polar auroras” – due to the planet’s magnetic field. Additional evidence of the oxygen atmosphere comes from the detection of various gases trapped in the ice on Ganymede. This evidence consisted of the spectroscopic detection of ozone (O³), as well as absorption features that indicated the presence of oxygen gas (O²). Another constituent of the atmosphere is hydrogen, which (despite most being lost to space) still exists at a surface in very minor concentrations. The existence of this neutral atmosphere implies that an ionosphere should exist since oxygen molecules are ionized by the impacts of the energetic electrons coming from the magnetosphere and by solar electromagnetic UV radiation.

However, the existence of an ionosphere remains controversial, due to conflicting data gathered by different missions. Magnetosphere: Ganymede is unique among moons in the Solar Systems in that it alone has a magnetosphere. The value of the moon’s permanent magnetic moment is estimated to be 1.3 x 10¹³ T·m³, which is three times larger than the magnetic moment of Mercury. The magnetic dipole is tilted with respect to the rotational axis of Ganymede by 176°, which means that it is directed against the Jovian magnetic moment. The magnetic field of Jupiter and co-rotation enforcing currents. Credit: Wikipedia Commons/Ruslik0 The dipole magnetic field created by this permanent moment has a strength of 719 ± 2 Teslas (nT) at Ganymede’s equator, and roughly twice that at the poles (1440 nT). This magnetic moment also carves a part of space around Ganymede, creating a tiny magnetosphere embedded inside that of Jupiter with a diameter of about 10,525 – 13,156 km.

The Ganymedian magnetosphere has a region of closed field lines located below 30° latitude, where charged particles (electrons and ions) are trapped, creating a kind of radiation belt. The main ion species in the magnetosphere is single ionized oxygen, which fits well with Ganymede’s tenuous oxygen atmosphere. The interaction between the Ganymedian magnetosphere and Jovian plasma is in many respects similar to that of the solar wind and Earth’s magnetosphere. The plasma co-rotating with Jupiter impinges on the trailing side of the Ganymedian magnetosphere much like the solar wind impinges on the Earth’s magnetosphere. In addition to the intrinsic magnetic moment, Ganymede has an induced dipole magnetic field who’s existence is connected with the variation of the Jovian magnetic field near Ganymede. The induced magnetic field of Ganymede is similar to those of Callisto and Europa, indicating that this moon also has a subsurface water ocean with a high electrical conductivity. Artist’s concept of aurorae on Ganymede. Credit: NASA/ESA However, the existence of Ganymede’s magnetosphere remains a bit of a mystery.

On the one hand, its existence is believed to be the result of Ganymede’s a dynamo effect caused by conducting material moving in the core, similar to Earth. However, other bodies that have differentiated metallic cores don’t have magnetospheres, and the relatively small size of Ganymede’s core suggests that it should have cooled sufficiently that fluid motions are no longer possible. One explanation for this incongruity is that the same orbital resonances that may have disrupted the surface also allow the magnetic field to persist. With tidal heating increasing during such resonances, the mantle may have insulated the core, preventing it from cooling. Another explanation is a remnant magnetization of silicate rocks in the mantle, which is possible if the satellite had a more significant dynamo-generated field in the past. Habitability: There is some speculation on the potential habitability of Ganymede’s ocean. An analysis published in 2014 , taking into account the realistic thermodynamics for water and effects of salt, suggests that Ganymede might have a stack of several ocean layers separated by different phases of ice, with the lowest liquid layer adjacent to the rocky mantle below.

This is important, since the layer closest to the rocky interior would be subject to heating due to tidal flexing in the mantle. This heat could be transferred into the water via hydrothermal vents , which could provide the necessary heat and energy to sustain life. Combined with oxygenated water, life forms could exist at the core-mantle boundary in the form of extremophiles, in a way that is similar to what is found in Earth’s oceans (and presumed to exist in Europa’s interior ocean). Exploration: Several probes flying by or orbiting Jupiter have explored Ganymede more closely, including four flybys in the 1970s, and multiple passes in the 1990s to 2000s. The first approaches were conducted by the Pioneer 10 and 11 probes, which approached the moon in 1973 and 1974, respectively. These missions returned more specific information on its physical characteristics and resolved features to 400 km (250 mi) on its surface.

The next missions came in 1979, when the Voyager 1 and 2 probes passed the moon, refining estimates of its size and revealing its grooved terrain for the first time. In 1995, the Galileo spacecraft orbited Jupiter and went on to make six close flybys between 1996 and 2000. The probe’s findings included the discovery of Ganymede’s magnetic field, the moon’s interior ocean, and a large number of spectral images that showed non-ice compounds on the surface. Artist impression of New Horizons conducting its flyby of Jupiter. Image credit: NASA/JPL/JHUAPL The most recent mission to Ganymede was made by the New Horizons probe in 2007. While en route to Pluto, the probe obtained topographic and composition mapping data of Europa and Ganymede during its flyby of Jupiter. Their are no missions to Ganymede currently in operation, but several missions have been proposed for the coming decades. One such proposal is the joint NASA/ESA Europa Jupiter System Mission (EJSM), which would explore Jupiter’s moons (including Ganymede) and has a proposed launch date of 2020.

The mission would consist of NASA’s Jupiter Europa Orbiter, the ESA’s Jupiter Ganymede Orbiter, and possibly a JAXA Jupiter Magnetospheric Orbiter. The ESA’s contribution was renamed the Jupiter Icy Moon Explorer (JUICE) in 2012, and obtained a launch slot aboard the ESA’s Cosmic Vision science program (scheduled for launch in 2022 or 2024). This may include a partner mission from the Russian Space Research Institute – known as the Ganymede Lander (GL) – and would involve JUICE examining Ganymede from orbit and conducting multiple flybys or Europa and Callisto. A Ganymede orbiter based on the Juno probe was also proposed in 2010 for the Planetary Science Decadal Survey. As part of the committee report presented at the Survey – titled “ Vision and Voyagers for Planetary Science in the Decade 2013-2022 ” – a concept study for a possible Ganymede Orbiter was proposed, which included recommendations on instrumentation.

Artist’s concept of the Jupiter Icy Moons Orbiter (JIMO), a cancelled program that envisioned sending a spacecraft to inspect Callisto, Ganymede and Europa. Credit: NASA/JPL A canceled proposal for a Ganymede orbiter was the Jupiter Icy Moons Orbiter (JIMO), which would have performed a flyby of Europa, Ganymede and Callisto. Designed to use nuclear fission for power and an ion engine for propulsion, JIMO would have studied Ganymede in greater detail than an previous orbiter. However, the mission was canceled in 2005 because of budget cuts. Colonization: Ganymede is considered a possible candidate for human settlement – and even terraforming – due to the many advantages it presents. For one, as Jupiter’s largest moon, Ganymede has a gravitational force of 1.428 m/s2 (the equivalent of 0.146 g) which is comparable to Earth’s Moon. Sufficient enough to limit the effects of muscle and bone degeneration, this lower gravity also means that the moon has a lower escape velocity – which means it would take considerably less fuel for rockets to take off from the surface.

What’s more, the presence of a magnetosphere means that colonists would be better shielded from cosmic radiation than on other bodies. The prevalence of water ice means that colonists could also produce breathable oxygen, their own drinking water, and would be able to synthesize rocket fuel. Unfortunately, beyond this, Ganymede presents numerous challenges for colonization. For starters, the presence of a magnetosphere does not shield Ganymede from enough cosmic radiation to ensure human safety, due to the fact that it is overshadowed by Jupiter’s powerful magnetic field. This results in the surface receiving some 8 rem of radiation per day – which is 333 times the average of what Earth-bound organisms experience in a year. Artist’s concept for a future settlement on Ganymede. Credit: futuretimeline.net The dominance of Jupiter’s magnetic field also means that Ganymede’s magnetosphere is not strong enough to retain an atmosphere of sufficient density to sustain human beings. It would also be insufficient to retain much heat. Hence, settlements on the surface would need to be heavily insulated, shielded from radiation, and contain a breathable atmosphere.

A possible solution to this, similar to what has been proposed for Europa, would be for colonists to build settlements within the icy mantle, or possibly under the surface of the ice entirely. These embedded (or aquatic habitats) would be shielded from harmful cosmic radiation by the icy mantle. They could also function as conduits between the ocean and the surface, piping water in and processing it into fuel for export. However, such possibilities are still far from realization and in the meantime, exploring Ganymede and deciphering its deeper mysteries remains the priority. And of these, Ganymede has several! Much like the other Galilean moons, Ganymede possesses a wealth of unique and mysterious attributes, many of which still defy comprehension. In addition to being the largest moon in the Solar System, it is the only moon other than Earth (and the gas giants) to have a magnetic field. And of course, there’s the possibility that life could exist beneath its icy crust, possibly in microbial or extreme form. All of these make Ganymede an intriguing prospect for future exploration. We have many interesting articles on Ganymede and Jupiter’s Moons here at Universe Today.

Here is a list of the Galilean Moons , as well as Jupiter’s entire system of moons . Here is one about Ganymede’s interior ocean , and how an amateur astronomer made a detailed map of Ganymede . Here’s a list of all the largest moons in the Solar System, and a listing of the largest moons and smallest planets at Solar Views. Universe Today has whole series of podcasts about the Solar System at Astronomy Cast . Share this: What is Jupiters Largest Moon | Space & Astronomy What is Jupiters Largest Moon · Jupiter Jupiter’s largest moon is the Ganymede. It is also the largest satellite in the Solar System. The largest moon of Jupiter is larger than Pluto and Mercury, and slightly smaller than Mars. Additionally, this moon can be misjudged as a planet if you are orbiting the sun, instead of Jupiter. Largest Moon of Jupiter The largest moon of Jupiter is about 4.5 billion years old, which is about the same age with Jupiter. Ganymede is the 7th moon on Jupiter and the 3rd Galilean satellite to be found outward from the planet.

It orbits around 1.070 million km or 665, 000 miles around Jupiter. Also, Ganymede takes seven Earth days to finish orbiting Jupiter. Ganymede’s radius is 2, 631.2 km or 1, 635 miles. Because of its size, it can be viewed on Earth with the naked eye. According to early astronomical records of the Chinese, the discovery of the moons of Jupiter started with the observation of Ganymede. Although the largest moon is larger than Mercury, Ganymede has only half the mass of Mercury and has a low density. The daytime temperatures of Ganymede on the surface average from 171F to -297F, and drop to -193C at night time. This is a clear indication that there is no any living organism that thrives in Ganymede. Ganymede Discovery The largest moon of Jupiter was discovered in January 7, 1610 by Galileo Galelei. He discovered Ganymede along with the three other Jovian moons. It was also the time when the moon is discovered to be orbiting the planet other than planet Earth.

His discovery of the Ganymede and Jovian moons led to the understanding that planets revolve around the sun, instead of the Solar System revolving around the Earth. Jupiter’s largest moon that time was not yet Ganymede. Galileo called this Jupiter III, but the numerical naming was abandoned on mid 1800s, so the moon was named after the Trojan prince in the Greek Mythology, Ganymede. The counterpart of Jupiter in Roman mythology is Zeus, who has been carried by Ganymede to Olympus in the form of a giant eagle. Ganymede Explorations There are human spacecrafts that flew to Jupiter and its moons. In 1973, Pioneer 10 arrived in Ganymede, followed by Pioneer 11 in 1974. Voyagers 1 and 2 have also taken pictures of Ganymede when they pass by the moon. Only the Galileo spacecraft has taken detailed images when it passed as low as 261 km or 162 miles over the Galilean moon’s surface. The atmosphere of Ganymede consists of thin oxygen, which is not capable of supporting life.

Also, Ganymede is the only satellite in the Solar System with a magnetosphere, aside from Earth. A magnetosphere is a comet-like region wherein charged particles are deflected or trapped. The magnetosphere of Ganymede is embedded with Jupiter’s magnetosphere. Also, 40% of Ganymede’s surface is covered with cratered dark regions. The remaining 60% of the moon’s surface is covered with a light grooved terrain, forming intricate patterns across the moon. Also, it has been believed that Ganymede has a 124 miles saltwater ocean below its surface. Jupiter's Smallest Known Moon Unveiled Jupiter's Smallest Known Moon Unveiled By SPACE.com Staff | June 12, 2012 11:46am ET MORE Image of S/2010 J 1 (circled), a 1.8-mile-wide moon of Jupiter discovered in September 2010. Credit: Palomar Observatory/University of British Columbia Astronomers have pinned down details of Jupiter's smallest known moon, a tiny space rock barely a mile across.

The moon, known as S/2010 J 2, was discovered in September 2010 along with a fellow shrimpy satellite called S/2010 J 1. S/2010 J 2 has a diameter of about 1.2 miles (2 kilometers), while S/2010 J 1 is about 1.8 miles (3 km) wide, researchers said. Their discovery brought the number of documented Jovian moons to 67. Scientists have kept observing the two satellites in the time since their initial detection, and a new study reveals key information about the moons and their orbits. For example, S/2010 J 1 is now known to circle Jupiter at an average distance of 14.57 million miles (23.45 million km), taking 2.02 years to complete a lap around the solar system's largest planet. S/2010 J 2 takes 1.69 years to zip around Jupiter, and its average distance is 13.06 million miles (21.01 million km).

The new observations also confirm the diminuituve dimensions of the two satellites, which researchers determined based on their brightness. "It was exciting to realize that this [S/2010 J 2] is the smallest moon in the solar system that was discovered and tracked from Earth," said co-author Mark Alexandersen, of the University of British Columbia, in a statement. Jupiter is surrounded by a veritable cloud of moons, with about 50 primary moons that have official names and at least 14 smaller provisional moons that are known only by their number, according to a NASA description. Scientists group the smaller, irregular satellites of giant planets such as Jupiter and Saturn in families with similar orbits and colors. These various families are believed to have formed when comets or asteroids smashed into larger moons long ago. S/2010 J 1 appears to belong to the Carme family, while S/2010 J 2 is likely a member of the Ananke group, researchers said.

Jupiter's moons are a very diverse group, ranging from the tiny, irregularly shaped S/2010 J 1 and S/2010 J 2 to the huge Galilean satellites Io, Europa, Callisto and Ganymede. With a diameter of 3,273 miles (5,268 km), Ganymede is the biggest moon in the solar system; it's larger than the planet Mercury. The new study, which details the discovery and tracking of S/2010 J 1 and S/2010 J 2, will appear this summer in The Astronomical Journal. Follow SPACE.com on Twitter @Spacedotcom . We're also on Facebook and Google+. JUPITER'S Moons - EnchantedLearning.com Jupiter's Moons Activities, Web Links JUPITER'S MOONS Jupiter has four large moons and dozens of smaller ones (there are about 67 known moons so far). Galileo first discovered the four largest moons of Jupiter, Io, Europa, Ganymede, and Callisto in 1610, using a 20-power telescope; these moons are known as the Galilean moons.

The moons of Jupiter are (in order by their distance from Jupiter): Metis, Adrastea, Amalthea, Thebe, Io, Europa, Ganymede (the biggest), Callisto (the second biggest), Leda (the smallest), Himalia, Lysithea, Elara, Ananke, Carme, Pasiphae, Sinope, and many newly-discovered moons that haven't been named yet. METIS Metis is the closest moon to Jupiter. Metis is 25 miles (40 km) in diameter and orbits 79,500 miles (128,000 km) from Jupiter, within its main ring. Metis and the next moon, Adrastea, are probably the source of the dust in this ring. Metis has a mass of 9 x 1016kg. It orbits Jupiter in 0.294780 (Earth) days; this is faster than Jupiter rotates on its axis. Metis was discovered by Stephen Synnott (Voyager 2) in 1980. ADRASTEA Adrastea is the second-closest moon to Jupiter.

Adrastea is 12 miles (20 km) in diameter and orbits 80,000 miles (129,000 km) from Jupiter, within its main ring. Adrastea and the first moon, Metis, are probably the source of the dust in this ring. Adrastea has a mass of 1.91 x 1016kg. It orbits Jupiter in 0.29826 (Earth) days; this is faster than Jupiter rotates on its axis. Adrastea was discovered by D. Jewitt and E. Danielson (Voyager 2) in 1979. AMALTHEA Amalthea is the third-closest moon to Jupiter and the reddest object in our solar system . Amalthea is 145 x 91 x 83 miles (232 x146 x134 km) in diameter and orbits 112,700 miles (181,300 km) from Jupiter, within the faint Gossamer ring. Amalthea and Thebe likely provide the dust for the Gossamer ring.

Amalthea has a mass of 7.2 x 1021kg. It orbits Jupiter in 0.49817905 (Earth) days and is in synchronous rotation (always keeping the same side facing Jupiter). Amalthea gives off more heat than it receives from the Sun. Amalthea was discovered by Edward Emerson Barnard in 1892. THEBE Thebe is the fourth-closest moon to Jupiter. Thebe is 68 x 56 miles (110 x 90 km) in diameter and orbits 138,000 miles (222,000 km) from Jupiter. Amalthea and Thebe likely provide the dust for the Gossamer ring. Thebe has a mass of 8 x 1017kg. It orbits Jupiter in 0.6745 day (Earth) days and is in synchronous rotation (always keeping the same side facing Jupiter). Thebe was discovered by Stephen Synnott (Voyager 1) in 1980. THE GALILEAN MOONS Two sulfurous eruptions on Io.

Pillan Patera, a volcanic caldera at the left, spews sulfur 86 miles above the surface. IO Io is a large, rocky, volcanically active moon of Jupiter. Its volcanoes spew out molten sulfur, making Io a very colorful moon. It is the innermost of Jupiter's four large moons and the third largest. It has a diameter of 1,942 miles (3,636 km), very close in size to our moon . Io's mean distance from Jupiter is 220,000 miles (422,000 km). It has a mass of 8.93x1022 kg. It takes Io 1.77 days to orbit Jupiter. There is a doughnut-shaped plasma cloud around Jupiter near Io's orbit (known as the "Io plasma torus") This torus is caused by Jupiter's strong magnetic field, which strips ions from Io as it rotates; Io acts like an electrical generator. Io was discovered by Galileo and Marius (independently) in 1610.

Ole Romer (1644-1710) was a Danish astronomer who, in 1675-1676, was the first person to demonstrate that the speed of light is finite. Romer did this by observing eclipses of Jupiter's moon Io as Jupiter's distance from Earth varied through the year. He noticed that the observed period of Io's orbit differed by about 20 minutes; he concluded that this difference was due to the extra distance that the light had to travel to Earth. His calculations put the speed of light at about 225,000 kilometers per second (it is really a bit faster, at 299,792 kilometers per second). EUROPA Europa is a large, dense, icy moon of Jupiter. Europa is the smoothest object in our Solar System . Its surface is covered with long, crisscrossing trackways (but few craters) on water ice. Frozen sulfuric acid has been found on its surface. Europa's diameter is less than 2,000 miles (3,138 km), smaller than the Earth's moon .

It takes Europa 3.55 days to orbit Jupiter (in a synchronous orbit). Its mean distance from Jupiter is about 420,000 miles (670,900 km). Its mass is 4.80x1022 kg. It was discovered by Galileo and S. Marius (independently) in 1610. GANYMEDE Ganymede is the largest moon of Jupiter, a large, icy, outer moon that is scarred with impact craters and many parallel faults. It has a diameter of about 3,400 miles (5,268 km) and orbits Jupiter at a mean distance of 664,000 miles (1,070,000 km). It has a magnetic field and probably has a molten iron core. It takes Ganymede 171.75 hours (7.15 Earth days) to orbit Jupiter. Its mass is 1.48x1023 kg. It was discovered by Galileo and S. Marius (independently) in 1610.

Ganymede is the largest moon in the solar system; it is also larger than the planets Mercury and Pluto. CALLISTO Callisto is a large, icy, dark-colored, low-density outer moon of Jupiter that is scarred with impact craters and ejecta. It has a diameter of about 3,000 miles (4800 km), the second-largest moon of Jupiter; it is roughly the size of Mercury . Callisto has the largest-known impact crater in the Solar System, Valhalla, which has a bright patch 600 km across and rings that go out to almost 3000 km. Callisto orbits Jupiter at a mean distance of 1,170,000 miles (1,883,000 km). Its mass is 1.08x1023 kg. It takes Callisto 400.8 hours = 16.7 days to orbit Jupiter (in a synchronous orbit). Callisto was discovered by Galileo and S. Marius (independently) in 1610. LEDA Leda is Jupiter's ninth and smallest moon.

Leda is 9.9 miles (16 km km) in diameter and orbits at an average of 6,900,000 miles (11,094,000 km) from Jupiter. Leda has a mass of 5.68 x 1015kg. It orbits Jupiter in 238.72 (Earth) days. Very little is known about Leda. Leda was discovered by Charles Kowal in 1974. HIMALIA Himalia is Jupiter's tenth moon. Himalia is 110 miles (170 km) in diameter and orbits 7,000,000 miles (11,480,000 km) from Jupiter. Himalia has a mass of 9.5 x 1018kg. It orbits Jupiter in 250.5662 (Earth) days. Very little is known about Himalia. Himalia was discovered by C. Perrine in 1904. LYSITHEA Lysithea is Jupiter's eleventh moon.

Lysithea is 15 miles (24 km) in diameter and orbits 7,200,000 miles (11,720,000 km) from Jupiter. Lysithea has a mass of 8 x 1016kg. It orbits Jupiter in 259.22 (Earth) days. Very little is known about Lysithea. Lysithea was discovered by S. Nicholson in 1938. ELARA Elara is Jupiter's twelfth moon. Elara is 50 miles (80 km) in diameter and orbits 7,250,000 miles (11,737,000 km) from Jupiter. Elara has a mass of 8 x 1017kg. It orbits Jupiter in 259.6528 (Earth) days. Very little is known about Elara. Elara was discovered by C. Perrine in 1905. ANANKE Ananke is Jupiter's thirteenth moon.

Ananke is 12.5 miles (20 km) in diameter and orbits 13,100,000 miles (21,200,000 km) from Jupiter. Ananke has a mass of 4 x 1016kg. It orbits Jupiter in 631 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction of Jupiter). Very little is known about Ananke. Ananke was discovered by S. Nicholson in 1951. CARME Carme is Jupiter's fourteenth moon. Carme is 18.5 miles (30 km) in diameter and orbits 13,800,000 miles (22,600,000 km) from Jupiter. Carme has a mass of 9 x 1016kg. It orbits Jupiter in 692 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction of Jupiter). Very little is known about Carme. Carme was discovered by S. Nicholson in 1938. PASIPHAE Pasiphae is Jupiter's fifteenth moon.

Pasiphae is 22 miles (36 km) in diameter and orbits 14,600,000 miles (23,500,000 km) from Jupiter. Pasiphae has a mass of 2 x 1023kg. It orbits Jupiter in 735 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction of Jupiter). Very little is known about Pasiphae. Pasiphae was discovered by P. Melotte in 1908. SINOPE Sinope is Jupiter's sixteenth moon. Sinope is 17.5 miles (28 km) in diameter and orbits 14,700,000 miles (23,700,000 km) from Jupiter. Sinope has a mass of 8 x 1016kg. It orbits Jupiter in 758 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction of Jupiter). Very little is known about Sinope. Sinope was discovered by S. Nicholson in 1914.

S/1999 J 1 (a provisional name) The seventeenth and outermost moon; S/1999 J 1 is the smallest-known moon orbiting a major planet. This moon is 3 miles (5 km) in diameter and has an irregular orbit roughly 15 million miles (24 million km) from Jupiter. It orbits Jupiter in 774 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction of Jupiter). It was discovered by Robert S. McMillan et al (at the Spacewatch program at the University of Arizona ) in 2000. The Galilean Moons of Jupiter The Galilean Moons of Jupiter Summary: Jupiter has more than 60 known moons, but understanding the geology of its four largest will hopefully lead to some groundbreaking discoveries. Sections: The Galilean Moons Each of the Jovian planets has a number of moons, although Jupiter has the most with more than 60 catalogued to date. Jupiter's 4 largest moons exhibit some of the most interesting geology in the solar system. They were discovered by Galileo Galilei and are known as the Galilean moons.

Jupiter's largest moon, Ganymede, is larger than Mercury while the other three are larger than Pluto. Outward from Jupiter: Io: Volcanoes and All Jupiter's Moon Io The Voyager spacecraft took the first close-up images of Io more than 300 years after the moon's discovery. The images showed a surface with no signs of craters from past impacts. What we saw instead was a surface almost entirely covered with large volcanoes. Cameras on Voyager actually captured volcanic eruptions in progress. The frequency of these sulfuric eruptions has filled in almost all of the impact craters and left Io with one of the youngest looking surfaces in the solar system. Io Close-Up (click to enlarge) Close-up photos of eruptions in progress show powerfully hot lava glowing orange and red. Photos taken on the night side of Io show not only the hot volcanic vents, but also a thin sulfur dioxide atmosphere produced by constant outgassing. Io's unusual red and orange colors come primarily from sulfur, which condenses on the surface after being outgassed by the volcanoes. Although there is no direct evidence of tectonic activity on Io, scientists feel confident it exists since the processes that fuel volcanism also fuel tectonics.

The volcanic eruptions are so frequent and cover the surface so thoroughly that any clear evidence of tectonic activity is likely to be buried. Europa: What Lies Beneath? Europa Europa's surface and crust are made almost entirely of water ice, and its bizarre, fractured appearance is proof enough that tidal heating has acted there. The icy surface is nearly devoid of impact craters and may be only a few million years old. Europa's Interior Observations made by the Galileo spacecraft show that Europa has a metallic core and a rocky mantle. Surrounding the rocky interior appears to be an icy layer 100 kilometers thick, the top few kilometers of which seem to be frozen solid. The stretching and squeezing of tidal friction should provide enough heat to melt some of this into liquid water beneath a thin ice shell. If it does, then Europa may have an ocean with more than twice as much liquid water as all of Earth's oceans combined. Analyzing Europa's Cracked Surface Close-up photos of the surface of Europa support the idea of a liquid ocean beneath the surface. These photos, taken by the Galileo spacecraft, show what appears to be icebergs stuck in a layer of ice. Other evidence comes from double-ridged cracks on the surface.

Tidal flexing that allows water to well up and build ridges may create these cracks. Ganymede: Largest Moon in the Solar System Two Regions of Ganymede's Surface (click to enlarge) The surface of Ganymede shares many similarities with Europa. Ganymede's surface is also made of water ice, but unlike Europa's surface, it shows signs of varying age. The darker regions are heavily cratered, suggesting they are billions of years old. The lighter regions show no signs of craters and it is thought that eruptions of water covered the surface before freezing over. These areas are geologically younger than the darker regions. If liquid water occasionally makes its way to the surface to fill in craters, could that suggest a liquid ocean similar to the one that might exist on Europa? Callisto: The Outermost Galilean Moon Callisto Callisto is the stereotypical outer solar system satellite. It is one of the largest and most heavily cratered satellites in the solar system. The surface is very icy and dates back four billion years. Beneath the icy crust is possibly a salty ocean supported by a deeper rocky interior.

Callisto's Surface Callisto doesn't have any large mountains, show evidence of volcanic or tectonic activity or have any appreciable level of internal heat. Nonetheless, observations of Callisto's magnetic field may cause scientists to add the large moon to the list of possible worlds with subsurface salty oceans.

Question: What is the name of Jupiter's largest moon?
Answer: Ganymede